The present invention relates to a new and improved apparatus for use in a vehicle to protect an electrical device against transient changes in voltage from a power source disposed in the vehicle.
During operation of a vehicle, such as an automobile or truck, relatively large transient voltages may occur. These transient voltages are of relatively short duration, less than 0.5 seconds, and may result from a loose battery connection or other causes. When electrical devices on the vehicle are exposed to the relatively large transient voltages, the devices could be damaged.
In the past, fixed voltage varistors have been used in vehicles to protect electrical devices against relatively large voltages. When the voltage being conducted to an electrical device becomes excessive, the resistance of the varistor drops and the excessive current is shunted away from the protected device. A varistor responds to changes in the absolute magnitude of the voltage from the vehicle power source. Therefore, if the magnitude of the voltage remained below a threshold voltage, a rate of change of voltage could occur without causing the varistor to shunt current away from the protected device.
Since the varistor responds to a predetermined voltage, the varistors provided in association with electrical devices in a vehicle must have characteristics which correspond to the normal magnitude of the voltage provided by the vehicle power source. Thus, a varistor used with a vehicle having a 12 volt power source must have characteristics which are different from the characteristics of a varistor used with a vehicle having a 24 volt power source. The use of different varistors for electrical devices used with different vehicles complicates the providing of electrical devices for vehicles having different power sources.